Elbow Anatomy and Biomechanics

Introduction

The elbow consists of three joints: the ulnohumeral joint, radiocapitellar joint, and proximal radioulnar joint (PRUJ). Together, these three joints enable the elbow to flex and extend as a hinge joint as well as rotate about a longitudinal axis for pronation-supination. As such, the elbow is classified as a trochoginglymoid joint.

The distal humerus has a 30-degree anterior tilt relative to the humeral shaft; therefore the anterior cortex of the humeral shaft should intersect the capitellum between the anterior and middle thirds of the capitellum. The trochlea is shaped like a spool of thread; the medial ridge of the trochlea is more prominent than the lateral ridge, and the trochlea extends more distal and posterior than the capitellum. As a result, the joint line is in about 6 degrees of valgus and 5 degrees of internal rotation. ^, This also contributes to the valgus carrying angle of the elbow, which is defined as the angle between the long axis of the humerus and the long axis of the ulna, and ranges between 10 and 15 degrees in males and 15–20 degrees in females. The distal humerus consists of two articulations: medially, the trochlea articulates with the greater sigmoid notch of the proximal ulna and laterally, the capitellum articulates with the radial head.

The ulnohumeral joint acts as a hinge joint. Anteriorly, the proximal aspect of the ulna is known as the coronoid process, which plays a critical role in elbow stability. It provides a highly congruent articulation, and several key soft tissue structures such as the medial collateral ligament (MCL), anterior capsule, and brachialis insert on the coronoid process. Posteriorly, the olecranon limits the extension of the elbow.

The radiocapitellar joint not only acts in flexion and extension but also allows for rotation. Here, the radial head articulates with the capitellum. The concave radial head provides stability at this joint through a concavity-compression method. Rotation occurs via the PRUJ. The PRUJ consists of the articulation between the radial head and the lesser sigmoid notch of the ulna. Approximately 240–280 degrees of the circumference of the radial head are covered with cartilage to allow for pronation and supination. ^{, , ,}

In the skeletally immature elbow, the ossification centers of the elbow appear in a predictable manner. The mnemonic “CRITOE,” or some variation, is often used to describe the pattern of ossification. The capitellum appears first, followed by the radial head, medial (“internal”) epicondyle, trochlea, olecranon, and lateral (“external”) epicondyle. On imaging, it is particularly important to examine the relationships between the anterior humeral line and capitellum and the radial head and capitellum to ensure appropriate reduction.

Muscular Anatomy

The elbow flexors consist of the biceps, brachialis, and brachioradialis. The biceps tendon attaches to the radial tuberosity, while the brachialis attaches 11 mm distal to the tip of the coronoid.

The triceps attaches to the olecranon and acts to extend the elbow. While the anconeus also contributes to elbow extension, it functions more as a restraint to varus and posterolateral rotatory instability (PLRI).

The forearm flexor-pronator group originates from the medial epicondyle of the elbow and consists of the flexor carpi radialis, palmaris longus, pronator teres, flexor digitorum superficialis (FDS), and flexor carpi ulnaris (FCU). The forearm extensors arise from a common origin of the lateral epicondyle and include the brachioradialis, extensor carpi radialis longus, extensor carpi radialis brevis, extensor carpi ulnaris, extensor digiti minimi, and extensor digitorum communis. The supinator also has one head that arises from the lateral epicondyle, while the other originates from the proximal ulna.